Opponents of a transportation sales tax on Missouri's August ballot filed a lawsuit Thursday asserting that the summary and cost estimate prepared for voters are misleading and unfair.

The lawsuit by the Missouri Association for Social Welfare asks a state judge to rewrite the ballot summary and order the state auditor to prepare a financial estimate to replace the one written by lawmakers.

Supporters of the ballot measure described the lawsuit as a long-shot attempt to make the wording more confusing.

The proposed constitutional amendment would impose a three-quarter-cent sales tax for roads, bridges and other transportation projects to run for the next decade. Lawmakers who placed the measure on the ballot project it will generate $534 million annually for state and local governments.

The lawsuit says the cost estimate is insufficient, in part, because it could be higher. It notes that Gov. Jay Nixon's administration has projected the sales tax to generate $6.1 billion over 10 years.

The lawsuit raises seven reasons why the ballot summary prepared by lawmakers should be considered insufficient, misleading or unfair.

It says the summary fails to mention that the measure also would raise the state use tax, which is levied on products used in Missouri but bought either from out-of-state retailers or from someone who does not run a business.

Among other things, the lawsuit says the summary should be changed to note that Missouri's current sales tax rate is 4.225 percent.

Instead of describing the proposal as a "temporary sales tax" for 10 years, the lawsuit contends the ballot summary should describe it as a tax that would last "indefinitely" unless rejected by voters after 10 years.

The Missouri Association for Social Welfare "fights for basic human needs and basic fairness for all Missourians, and this ballot measure fails in regard to both," said executive director Jeanette Mott Oxford, who is a plaintiff in the lawsuit.

Over the last decade, it's become common for opponents of Missouri ballot measures to file lawsuits contending the summaries are insufficient or unfair.

Some of those lawsuits have succeeded in forcing changes, but many have not.

Bill McKenna, a former state transportation commissioner who is treasurer of a committee supporting the sales tax, described the lawsuit as a "long-shot attempt" that "borders on being frivolous." He said the ballot wording already is simple and clear.

"This is a coordinated effort to confuse voters, and courts do not favor such efforts," McKenna said.

The lawsuit was filed on behalf of the social welfare group by Jefferson City attorney Gaylin Rich Carver. One of the attorneys listed on her law firm's website is Georganne Wheeler Nixon, the wife of the governor, who announced his opposition to the transportation tax last week.